1. Field of the Invention
The present invention relates generally to a control system for an automotive vehicle which controls vehicular activity for achieving improved vehicle driving performance with satisfactorily high driving stability and riding comfort. More specifically, the invention relates to an automotive control system employing an acceleration sensor, such as a lateral acceleration sensor, a longitudinal acceleration sensor, a vertical acceleration sensor and so forth, and a system for monitoring operation of the acceleration sensor in order to detect failure for alarming and/or taking place fail-safe operation.
2. Description of the Background Art
In the recent years, various electronic control systems have been developed, such as an anti-skid brake control system for controlling braking pressure in an automotive brake system for optimizing vehicular braking performance, power distribution control system for adjusting distribution of driving torque for front and rear wheels, a suspension control system for controlling suspension characteristics for effectively absorbing road shock and controlling attitude of the vehicular body. Such control systems employ various acceleration sensors, such as a lateral acceleration sensor monitoring acceleration magnitude exerted on the vehicular body in a direction lateral to a longitudinal axis of the vehicular body, a longitudinal acceleration sensor for monitoring acceleration magnitude exerted along the longitudinal axis, a vertical acceleration sensor and so forth. Control systems employing such acceleration sensors have been employed in Japanese Patent First (unexamined) Publications (Tokkai) Showa 51-6305, 61-169326 and 63-130418.
The vehicular activity control systems, such as that listed above, take accelerations, e.g. lateral acceleration, longitudinal acceleration, vertical acceleration and so forth, as one of typical and essential control parameters. Therefore, failure of acceleration sensors providing such control parameters tends to result in failure of control. Concerning type of failures possibly caused in the acceleration sensors, there are two types of possible failures to be caused. One of the failures is so-called "zero-output failure" in which the acceleration sensor outputs acceleration indicative signals representative of acceleration magnitude of zero despite noticeable magnitude of acceleration. The other is so-called "abnormal-output failure" in which the acceleration sensor outputs abnormal value of acceleration indicative signals irrespective of actual magnitude of acceleration. The former zero-output failure may not cause a substantial problem in vehicular activity control though it naturally causes degradation of control performance in a certain level. On the other hand, abnormal-output failure may result in malfunction of the control system and thus cause serious problems.
In the existing technologies, abnormal-output failure of the acceleration sensors has been detected by comparing the sensor outputs with predetermined reference values which represent possible maximum accelerations. Namely, in case of the automotive control systems, the possible maximum longitudinal acceleration magnitude may not exceed road/tire friction upon braking; the possible maximum lateral acceleration may be exerted during cornering. Therefore, by setting the possible maximum longitudinal acceleration at the road/tire friction and the possible maximum lateral acceleration at the lateral acceleration at the cornering, abnormally great acceleration indicative sensor outputs can be detected. Such conventionally employed failure monitoring technologies are not at all complete because they can not detect abnormal-output failure when failure occurs at acceleration magnitude far below the aforementioned possible maximum value as the reference level.